The Atypical Hysteresis of [Fe(C6F5Tp)2]: Overlay of Spin‐Crossovers and Symmetry‐Breaking Phase Transition

The [FeII(C6F5Tp)2] spin‐crossover complex is an atypical molecular switch, which can be converted upon annealing between two archetypal spin‐crossover behaviours: from an extremely gradual spin‐crossover to a broad hysteretic spin‐transition (of ca. 65 K). The hysteresis shows an uncommon “rounded shape” that is reproducible upon cycling temperature. In depth structural studies reveal a first crystal phase transition, which occurs upon melting and recrystallizing at high temperature. This first irreversible transition is associated with a radical change in the crystal packing. More importantly, the “rounded and broad” hysteretic transition is shown to occur in a non‐cooperative SCO system and is associated with the occurrence of a symmetry‐breaking phase transition that appears when roughly ca. 50 % of the SCO complexes are switched. The [FeII(C6F5Tp)2] complex is converted through melting from a crystal phase showing a very gradual spin‐crossover to another one showing a broad hysteretic spin‐transition. The uncommon “rounded” hysteresis occurs in a non‐cooperative SCO system. It is due to a symmetry‐breaking phase transition that is triggered when roughly ca. 50 % of the SCO complexes are switched.